[docs] Message API

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[andrewnicols]

Thanks for your change here.

The changes look pretty good. I've just added a commit to your branch to:

• correct some typos
• update coding style to meet current coding style

We can do more to improve this documentation but we'll do so in a different issue.

Thanks for your contributions!

[Arjun-A3]

hey,I am new to contributing,
I am trying to contribute by migrating css-coding-style but I can't decide on what to write on CSS.md
did you write the contents in docs/apis/core/message/index.md by yourself or referred from any specific docs?
if so kindly comment on the docs please
thank you.

#697